1. Field of the Invention
This invention relates to a crystal monochromator, such as a bent Laue crystal monochromator, which diffracts a large area of divergent monochromatic beams or rays, such as X-rays.
2. Description of Prior Art
Conventional angiography systems use polychromatic X-rays and intra-arterial injection of contrast agents. Dual-energy subtraction imaging with intravenous injection of the contrast agent can produce usefuil images with much reduced risk. Early attempts at intravenous angiography with non-synchrotron X-rays included the use of filtered or kVp-modulated polychromatic X-rays and dual-energy subtraction methods. The broad spectra of the X-rays used by conventional methods requires three energies in order to minimize bone artefacts. Prior synchrotron-based patient studies using the dual-energy digital subtraction intravenous coronary angiography technique with monochromatic X-rays have obtained research quality images of the coronary artery anatomy. However, the cost of a synchrotron prevents its general use for clinical diagnostic imaging. Development of a compact source and a corresponding X-ray optics system is necessary for the technology to be widely utilized.
One of the recent developments of compact sources which would have sufficient intensity for digital subtraction coronary angiography is an X-ray generator with a rotating anode coated with barium and cerium. In addition to the desired characteristic X-rays, the bremsstrahlung radiation from the source is also present. This continuum in the emitted X-ray spectrum increases the dose to a patient, creates subtraction artifacts due to beam hardening effects, reduces contrast and adds noise to the subtracted image.
Medical imaging with monochromatic beams produced with synchrotron X-rays and crystal monochromators show significantly improved image quality compared to conventional methods in several fields, including transvenous coronary angiography, mammography and computed tomography. However, the use of synchrotron radiation for clinical applications may not become widespread due to the synchrotron size, cost and complexity of operation.
The development of compact sources of narrow energy-band X-rays for radiography has been the subject of several studies in recent years. One such proposal is for the use of a rotating anode X-ray source for digital subtraction coronary angiography.
The source utilizes a high-energy (up to 1 MeV) electron beam in conjunction with selected rare-earth anodes. Anode materials can be chosen so that their characteristic emission lines bracket the iodine K absorption edge. The source provides adequate beam intensity for digital subtraction imaging of the coronary arteries with an iodine contrast agent delivered intravenously. In that particular system design, however, the resulting energy bandwidth is not narrow because the beam, along with the characteristic X-rays, includes a substantial amount of bremsstrahlung radiation. The bremsstrahlung continuum increases noise to the subtracted image.